In recent years, along with an increased demand for thinner TVs, there have been developed display technologies for various systems such as liquid crystals, plasma, organic electroluminescence, and field emission. The transparent electrode is incorporated, as an essential constitutional component, in any of the displays with a different display technology system. Further, the transparent electrode has been an indispensable component in touch panels, cellular phones, electronic papers, various solar cells and various electroluminescence controlling elements, in addition to TVs.
Hitherto, as a transparent electrode, there has been mainly used an ITO transparent electrode having an indium-tin complex oxide (ITO) membrane produced by a vacuum deposition method or a sputtering process on a transparent base material such as glass or a transparent plastic film. However, the transparent electrode produced by a vacuum deposition method or a sputtering process is poor in productivity, high in production cost, and inferior in flexibility, and therefore, has problems in that it cannot be applied to devices in which flexibility is required.
In contrast, a method has been proposed which forms a transparent electrode by coating of particles of a metal such as ITO (for example, refer to Patent Document 1), but has problems in that conductivity enough to work as an electrode cannot be obtained. Calcination treatment of a metal oxide particle coating layer makes it possible to lower the resistance of the layer, however, the treatment cannot be applied to a transparent electrode which is formed on a resin substrate such as a plastic film.
As a method of forming a transparent electrode excellent in productivity, there has been proposed a method of form a transparent electrode by coating or printing of a coating liquid prepared by dissolving or dispersing conductive polymer materials represented by π-conjugated polymers in an appropriate solvent (for example, refer to Patent Document 2). However, this electrode has problems of lower electrical conductivity and poor transparency, as compared to a transparent electrode of a metal oxide such as ITO, which is prepared by a vacuum film formation method.
Further, a technology has been disclosed which employs conductive fibers such as carbon nanotubes (CNT) or metal nanowires. Furthermore, a technology has been proposed which achieves compatibility between high transparency and high conductivity by an electrode in which a part of conductive fibers is fixed onto a substrate by a transparent resin film and another part of conductive fibers protrudes or an electrode in which conductive fibers in the form of nanowires and a conductive polymer are present in the same layer (for example, refer to Patent Documents 3 and 4). However, this technology has problem in that there is a limit to achievement of compatibility between high transparency and high conductivity, and problem in poor durability and in short product lifetime.
A detailed study has not hitherto been made on a purifying method of conductive fibers. A purifying method of separating conductive fibers as impurities from metal nanowires has been proposed (for example, refer to Patent Document 5). However, when filtration is employed as a purifying method which removes minute amount of impurities from the conductive fibers, the purification is difficult because of the specific structure of conductive fibers with very high aspect ratio. A purifying method in which a minute amount of impurities was removed from the conductive fibers has been desired.